Many current wheelchairs have a well-known four wheel design, in which a pair of large rear wheels are propelled by the user and two smaller front castors facilitate maneuvering. Whilst satisfying many of the requirements of users, this arrangement of wheels is not ideally suited to anything other than smooth surfaces, since the front castors, which are limited by the layout to be small in diameter, have difficulty overcoming obstacles such as gravel, sand and small rocks encountered in off-road conditions. In addition, in uphill as opposed to downhill conditions the weight distribution of the user about the chair is altered, which may lead to an unstable condition. There is also a risk that conventional wheelchairs can tip backwards when the user pushes hard on the wheels in order to accelerate.
In some alternative wheelchair designs, the drive wheels and the castor wheels have been reversed, such that the two large propelling wheels are at the front and the two castors are rearmost. This provides a more satisfactory solution to negotiating off-road conditions, and has reduced the tendency of the wheelchair to tip backwards under acceleration.
However, as the surfaces encountered by a wheelchair user, particularly off-road, are not likely to be smooth and level, a four-wheeled wheelchair of the front-drive design can find itself with only three of the wheels in contact with the ground and one wheel raised clear. If one of these wheels is a driving wheel, the user may become immobilised, requiring assistance. This has led to the development of three-wheeled wheelchairs, having two large propelling wheels at the front and a single trailing castor wheel at the rear. The single trailing castor prevents the wheelchair from tipping backwards when accelerating forwards. Being three-wheeled, such a wheelchair has all three wheels in contact with the ground, whatever the unevenness of the terrain.
However, it has been found that existing three-wheeled wheelchairs are sometimes highly unstable, particularly in off-road conditions and when traversing inclines or stopping on inclines. In such cases, the three-wheeled wheelchair has been known to topple over, which can cause an accident or an injury to the user. Such problems are believed to be a consequence of the relatively high centre of gravity of a user when using a three-wheeled wheelchair.
The driving wheels (i.e. the wheels that are propelled by the user's hands) of a wheelchair are sometimes cambered. This cambering, whereby the wheels are angled such that the bottom of each wheel is further out from the body of the wheelchair than is the top of each wheel, is often done to enhance the stability of the wheelchair and to place the top of the driving wheels within easier reach of the user's hands. However, a problem experienced by users of such wheelchairs is that the width of the wheelchair across the driving wheels is sometimes too large to enable the wheelchair to fit through doorways, between furniture or past obstacles.
A further problem experienced by wheelchair users is that, when ascending a slope or incline, the wheelchair has a natural tendency to roll backwards. This may happen when the user releases their hands from the driving wheels, between pushes, or if the user wishes to stop on the incline for some reason. This tendency of a wheelchair to roll backwards may lead to accident or injury to the user or to a bystander, particularly if the wheelchair runs backwards out of control and/or topples over.
Finally, a further problem experienced by wheelchair users is that it can be awkward assembling a collapsible or disassemblable wheelchair, particularly if the user has use of only one hand. Thus, there is a desire to be able to join structural members easily, preferably using only one hand, but nevertheless such that the members are reliably joined. It will be appreciated that there is also a desire to be able to readily disassemble components, again preferably with only one hand.